The invention relates generally to transistor switches, and more specifically to transistor switches used for high frequency and high power applications.
Typically, in power electronic applications, it is desirable to operate at high switching frequencies especially in motor control and switch mode power supplies. For most of the high switching frequency applications in power circuits, it is generally required to use power devices with improved switching performance.
Gas discharge switches can be used for high frequency pulsed power applications. A few disadvantages of gas discharge switches are low repetition rates, short service lifetimes, weight and size. Such disadvantages can be overcome by using semiconductor switches. Semiconductor based switches typically have lower power dissipation, longer life, fast turn-on and turn-off, high blocking voltage and improved current handling capability.
PiN bipolar rectifiers are typically used in power circuits for rectification and as anti-parallel diodes for switches such as insulated gate bipolar transistors (IGBT) and metal oxide semiconductor field effect transistors (MOSFET). One limitation of such devices operating at high switching frequencies is the reverse recovery process when a large reverse transient current flows through the device thereby increasing the diode power dissipation and producing an undesirable stress upon the power transistors operating in the circuits. Other rectifiers such as Silicon Schottky rectifiers on the other hand exhibit poor, reverse blocking characteristics due to the Schottky barrier lowering effect and the large forward voltage drop that results when designed for high blocking voltage.
Power bipolar transistors are also used for high switching frequency and medium power applications. Most bipolar transistors are current controlled devices and a large reverse base drive current is often needed to get a fast turn-off. Such devices are prone to second breakdown failure mode under simultaneous application of high current and high voltage as usually encountered in inductive power circuits.
Static induction transistors can also be used for high switching frequency applications. Charge transport in such transistors is due to majority carriers (for example electrons) flow through the channel, which is controlled by a channel potential barrier “induced” by a drain-source and a gate-source potential. Such transistors are typically vertical channel structures with uniform doping in the channel region. On proper scaling of such devices, large current handling capacity and low power dissipation in the on-state can be achieved. One problem with such transistors when made in silicon is their inability to withstand high blocking voltage because of low bandgap energy.
It is therefore desirable to design a transistor switch that is suitable for operating in high switching frequency as well as withstanding high blocking voltages.